1. Field of Invention
The present invention generally relates to self-reinforced composites and methods for their manufacture and, in particular, to self-reinforced composites which incorporate recycled thermoplastics.
2. Background of the Invention
Environmental and societal urgency to reduce landfill waste from the carpet and automotive industries is emerging. According to the Environmental Protection Agency, the municipal solid waste generated in the USA is over 245 million tons per year, where about 13% are synthetic polymers, including plastics, rubbers, and carpet and textiles. Most synthetic polymer wastes are derived from immiscible multiple polymer systems and are thus difficult to recycle. At present, only about 6% plastics, 19% rubber, and 1% carpet is recovered for recycling. The remaining is either landfilled or incinerated for energy recovery. Among these figures, over 5 billion pounds of carpet were discarded and landfilled in the USA in 2006.
Being the main method in solid waste handling, landfilling appears to be a rather ineffective method, particularly for synthetic polymer wastes. Unlike natural polymers, such as cellulose in cotton, synthetic polymers, e.g., nylon, polypropylene (PP), polyesters, and others, are difficult to degrade after landfill, thus polluting the land for many years. Further, since these polymers are derived from petroleum, a limited natural resource, continuous landfilling rather than recycling would ultimately deplete the world storage. It is therefore highly desired to reclaim polymers from the waste and to reuse them in new products.
Most polymer-based products are composed of more than one polymer. For example, over 80% carpet is made of nylon face fibers and PP backings. Multiple polymers are also widely used in automotive components. A car bumper is commonly made of a primary structure (e.g. nylon) and a secondary impact absorbing material (e.g. PP foams). The soft touching material used in seating and roofing typically comprises a polycarbonate film coated with a soft polyurethane layer and backed by a polyurethane foam or a foam made of a third polymer. Considering the large number of different grades of polymers used in end products and the complexity in identifying and sorting them, polymer recycling is difficult. The process becomes further complicated because most polymers are immiscible or incompatible at the molecular level. The nature of immiscibility greatly limits the use of them, mainly due to their poor mechanical properties. For example, nylon and PP, commonly used in carpet and automotive applications, are highly immiscible because of their different polarity. For these reasons, most efforts in polymer recycling have been focused on developing methods for separating and identifying different polymers from the waste.
While many recycling centers currently in operation still largely rely on labor-intensive manual sorting, the industry has introduced a number of techniques, including near-infrared spectroscopy, artificial intelligence, triboelectric separation, sink-float procedures, and the use of powder hydrocyclones for identification. These methods have proven to be valuable in sorting discrete waste articles, e.g. plastic bottles. However, they are difficult to implement in separating polymer wastes involving immiscible polymers. The reason becomes evident considering the fact that different polymers in the immiscible polymers waste are typically interlocked or bonded intimately. Carpets and automotive-used soft touching materials, as mentioned above, are among such products. To prepare for sorting, one needs to shred the immiscible polymer waste into particles fine enough to mechanically disintegrate the different polymers. This is not only highly energy consuming, but also difficult or even impossible to practice in many cases.
An alternative method [U.S. Pat. No. 7,067,613; U.S. Pat. No. 5,849,804] for separating immiscible polymers is solvent extraction. Such a method involving potentially hazardous solvent handling is hard to implement in large scale.
Some previous attempts [U.S. Pat. No. 7,173,127; U.S. Pat. No. 5,532,404; U.S. Pat. No. 5,169,870] have been made by depolymerizing the face fiber of the post consumer carpet waste into a monomer. In the case of nylon, it can be depolymerized by hydrolysis or ammonolysis. Following purification, the monomers may be re-polymerized to form new polymers having similar performance characteristics to those of virgin polymers. The design and setup of these chemical processes, however, need to be customized based on the source and composition of waste and are thus expensive to implement. According to a recent reviewing report in nylon recycling, depolymerization-based recycling business for nylon carpet almost vanished by the end of 2004.
Other processes focus on recycling immiscible polymer wastes by melt blending. The success of this method, however, is greatly hampered by the immiscible nature of polymers, resulting in end-products having very poor mechanical properties. Some processes [U.S. Pat. No. 5,591,802; U.S. Pat. No. 5,294,384] teach to improve the mechanical properties of the end products by the addition of compatibilizers during melt blending. One drawback of the compatibilization method is the high cost of the compatibilizer. Further, for different wastes with different polymers involved, different compatibilizers are needed. Even with compatibilizers, the mechanical performance of the PP/nylon blend reported so far is still poor. In addition to adding the compatibilizers, other processes [U.S. Pat. No. 6,756,412, U.S. Pat. No. 6,271,270] focus on adding high modulus fiber such as glass fiber and a fiber adhesion promoter to reinforce the recycled composite containing nylon and PP. These methods involve an addition of a high cost foreign reinforced material such as glass fiber in a separated step to improve mechanical properties. Also, the addition of the foreign reinforced fiber further increases the weight of the composite which is undesirable in most of the applications.
All of these prior attempts for recycling of thermoplastic waste consisting of immiscible polymers suffers from shortcomings, thus an unmet need exists for an environmentally benign, inexpensive, and practical method of recycling immiscible polymers.